1. Field of the Invention
The invention relates to a method of converting a series of m-bit information words to a modulated signal, with m being an integer, in which method an n-bit code word is delivered for each received information word, and the delivered code words alternate with p-bit merging words converted to the modulated signal, and (n+p) exceeding m, and in which the series of information words is converted into a series of alternate code words and merging words according to rules of conversion, so that the corresponding modulated signal satisfies a predetermined criterion. The invention further relates to a method of producing a record carrier on which a signal is recorded obtained according to said method.
The invention further relates to a coding device for performing the method as claimed, this device comprising an m-to-n bit converter for converting the m-bit information words to n-bit code words, and means for selecting the p-bit merging words, and means for converting the alternate n-bit code words and p-bit merging words into a modulated signal.
The invention further relates to a recording device in which a coding device of this type is used.
The invention further relates to a signal.
The invention further relates to a record carrier on which the signal is recorded.
The invention further relates to an apparatus for manufacturing said record carrier, comprising an optical system for scanning a radiation-sensitive layer of a record carrier by a radiation beam and a modulation unit for modulating the radiation beam in such a way that the pattern formed by the radiation beam in the radiation-sensitive layer corresponds to a control signal applied to the modulation unit.
The invention further relates to a decoding device for converting the signal to a series of m-bit information words, this device comprising converting means for converting the signal to a string of bits having a low or high logical value, this bit string containing n-bit code words which correspond to the information signal portions, and this device comprising converting means for converting the series of code words to the series of information words, while a code word-dependent information word is assigned to each of the code words to be converted.
Finally, the invention relates to a reading device in which a decoding device of this type is used.
2. Description of the Related Art
Such methods, such devices, such a record carrier and such a signal is known from the book xe2x80x9cCoding Techniques for Digital Recordersxe2x80x9d by K. A. Schouhamer Immink, Chapter 5, Prentice-Hall, 1991, ISBN 0-13-140047-9. In said title, for example, encoders are described, which are used for converting a series of m-bit information words into a series of bits wherein the number of xe2x80x9c0xe2x80x9ds between two consecutive xe2x80x9c1xe2x80x9ds lies between d and k. These constraints are also referenced d- and k-constraint, or dk-constraint. In specific prior art methods, (see Pages 114-117 of said title) m-bit information words are converted into n-bit code words in accordance with a conversion table, and where p-bit merging words are inserted between consecutive code words, m, n, p being integers, (n+p) exceeds m, and p exceeds or equals d. The code words and the merging words are chosen such that the dk-constraint of the catenation of alternate code words and merging words is satisfied. The alternate code words and merging words are converted, via a modulo-2 integration operation, into a corresponding signal formed by bit cells having a high or low signal value, a xe2x80x9c1xe2x80x9d-bit being represented in the modulated signal by a change from the high to the low signal value or vice versa. A xe2x80x9c0xe2x80x9d-bit is represented by the absence of a change of signal value at a transition between two bit cells. The minimum distance between consecutive transitions of the modulated signal is d+1 bit intervals and the maximum distance between consecutive transitions of the modulated signal is k+1 bit intervals. The dk-constraint is imposed as it is desirable that the system is self-clocking, which requires that consecutive transitions in the modulated signal should not be too far apart, and it is a further requirement that two transitions of the modulated signal should not be following too closely in order to limit inter symbol interference. In addition, the low-frequency components of the modulated signal should be kept as small as possible. Such a signal is also called a dc-free signal. A first reason for using said dc-free signals is that recording channels are not normally responsive to low-frequency components. The suppression of low-frequency components in the signal is also highly advantageous when the signal is read from an optical record carrier on which the signal is recorded in the track, because then continuous tracking control undisturbed by the recorded signal is possible. A good suppression of the low-frequency components leads to improved tracking with less disturbing audible noise.
An example of the use of such signals to record and read an audio signal on an optical or magneto-optical record carrier can be found in U.S. Pat. No. 4,501,000. The specification describes the EFM modulation system, which is used for recording information on Compact Discs (CD) or MiniDisc (MD). The EFM-modulated signal is obtained by converting a series of m(=8)-bit information words into a series of n(=14)-bit code words, and where p(=3) merging bits are inserted between consecutive code words. Respective code words of 14 bits satisfy the conditions that at least d(=2) and at most k(=10) xe2x80x9c0xe2x80x9ds are placed between two consecutive xe2x80x9c1xe2x80x9ds. In order to satisfy this condition also between code words, 3-bit merging words are used. Four 3-bit merging words of the 8 possible 3-bit merging words are permitted to be used, namely xe2x80x9c001xe2x80x9d, xe2x80x9c010xe2x80x9d, xe2x80x9c000xe2x80x9d, and xe2x80x9c100xe2x80x9d. The remaining possible 3-bit merging words, namely xe2x80x9c111xe2x80x9d, xe2x80x9c011xe2x80x9d, xe2x80x9c101xe2x80x9d, and xe2x80x9c110xe2x80x9d are not used as they violate the prescribed d(=2)-constraint. One of the four allowed merging words is selected such that the bit string obtained after cascading alternate code words and merging words satisfies the dk-constraint, and that in the corresponding modulo-2 integrated signal the running digital sum value remains substantially constant. The running digital sum value, RDS, at a specific instant is understood to mean the difference between the number of bit cells having the high signal value and the number of bit cells having the low signal value, calculated over the modulated signal portion situated before this specific instant. A substantially constant running digital sum value means that the frequency spectrum of the signal does not comprise frequency components in the low frequency area. By deciding the merging words according to above rules, low-frequency components of the modulated signal can be reduced. The choice for the 3-bit merging words is based on the requirement that, on the one hand the channel signal is substantially dc-free and that also the dk-constraint for the channel signal is satisfied. Decoding of EFM signals is very simple. The 3-bit merging words are skipped by the decoder, and the 14-bit code words are translated, using a look-up table or PLA etc., into the information bytes.
An improved method for suppressing the low-frequency components is described by K. A. Schouhamer Immink and U. Gross in the article entitled xe2x80x9cOptimization of Low-frequency Properties of Eight-to-Fourteen Modulation (EFM)xe2x80x9d, published in the Radio and Electronic Engineer, vol. 53, pp. 63-66, 1983. In said article the authors describe a method, where the selection of a p-bit merging word does not only depend on a single upcoming code word, but where in contrast the selection is made using q upcoming code words, where q is an integer exceeding 1. From the article cited, it can be concluded that this so-called look-ahead strategy improves the quality of the low-frequency suppression. A significant disadvantage of said strategy is the difficulty of implementing it as the number of operations, such as adding, comparing, buffering, and so on, grows exponentially with the number of code words involved in the selection process. By way of example, a look-ahead strategy using two code words requires 4xc3x974=16 add/compare operations, and a strategy using three code words requires 4xc3x974xc3x974=64 add/compare operations, and so on. For many applications, where speed and power consumption of the selection circuitry is at premium said look-ahead strategy cannot practically be employed.
In a recent article entitled xe2x80x9cWeakly constrained codesxe2x80x9d, published in IEE Electronics Letters, vol. 33, no. 23, pp. 1943-1944, November 1997, the author, K. A. Schouhamer Immink, described a novel type of constrained codes, termed weakly constrained codes. Weakly constrained codes fail to comply with the dk-constraints as they produce sequences that violate the prescribed constraints with (low) probability P. The author argues that if the channel is not free of errors, it is pointless to feed the channel with perfectly constrained sequences. Violation of prescribed dk-constraints offers an additional degree of freedom that can be exploited to reduce the low-frequency components. It is known that violations of the d-constraint, i.e., runs of xe2x80x9c0xe2x80x9ds smaller than d, are prone to error resulting from inter symbol interference. A violation of the k-constraint could easily result in loss of clock synchronization, which, in turn, could lead to a burst of errors The method is therefore not very effective as the frequency of violations of the dk-constraint must be small as they will inevitably lead to bit detection errors at the receiver""s site
Information recording has a constant need for increasing the reading and writing speed. The aim of increased reading speed, however, requires higher servo bandwidth of the tracking mechanism, which, in turn, sets more severe restrictions on the suppression of the low-frequency components in the recorded signal. Improved suppression of the low-frequency components is also advantageous for suppressing audible noise arising from the tracking mechanism. For this reason, it is desirable to make as many efforts to prevent the signal from containing low-frequency components.
It is, therefore, an object of the invention to provide a digital modulation method, which is capable of suppressing the low-frequency components with high efficiency, while achieving this goal with a small computational load such as add/compare operations.
A further object of the invention is to provide an information-recording medium such as an optical or magneto-optical disc on which digital data modulated by the invention process is recorded.
A possible solution to the problems inherent in the prior art is based on the observation that prior art methods are overly restrictive in the choice of the merging words as only those merging words are allowed that when alternate merging words and code words are cascaded the prescribed dk-constraint is satisfied.
In a first aspect of the present invention there is provided a digital modulation method for converting m-bit information words into n-bit code words, and where p-bit merging words are inserted between consecutive code words, m, n, p being integers, where (n+p) exceeds m. The code words obey both d- and k-constraint in force, but the merging words are not required to obey said d-constraint. It is known that runs of xe2x80x9c0xe2x80x9ds smaller than d are prone to error resulting from inter symbol interference. However, as the merging words do not provide information to the receiver, the d-constraint in the merging words can be violated without compromising the reliability of the received code words. Then, as a result of the increase of the size of the set of merging words from which can be selected, the low-frequency components of the modulated signal can significantly be reduced with respect to modulated signals generated under the rules of prior art methods or apparatuses. This embodiment is advantageous in that the presence of low-frequency components in the modulated signal can be significantly better avoided than in the prior art.
A further embodiment of the invention is characterized in that synchronization (sync) words are inserted into the series of code words, the sync words showing bit patterns that cannot occur in the bit stream formed by the catenation of the alternate merging words and code words. The selection circuitry excludes those p-bit merging words that when catenated with the alternate code words would generate the sync pattern. The above excluding rule treats the p-bit merging words that contain u xe2x80x9c0xe2x80x9ds between consecutive xe2x80x9c1xe2x80x9ds, where u is an integer less than d, as the all xe2x80x9c0xe2x80x9ds word. This embodiment is advantageous in that in the case the reading mechanism might detect the short xe2x80x9c0xe2x80x9d runlength as the all xe2x80x9c0xe2x80x9ds word, false sync patterns are not generated.
These and other objects, features, and advantages of this invention will become more fully apparent from the ensuing description of preferred embodiments of this invention, wherein